1. Field of the Invention
The present invention relates to a thin film transistor (TFT) using an oxide semiconductor layer, and a display device including the same.
2. Description of the Related Art
An oxide semiconductor composed of zinc oxide, indium gallium zinc oxide (IGZO), or the like exhibits excellent characteristics as an active layer of a semiconductor device, and development of the oxide semiconductor has been in progress toward application to a TFT, a light emitting device, a transparent conductive film, and the like in recent years.
For example, the TFT using the oxide semiconductor has high electron mobility, and excellent electric characteristics in comparison with the TFT using amorphous silicon (a-Si:H) as a channel, which has been used in the existing liquid crystal display device. Moreover, the TFT using the oxide semiconductor has an advantage that high mobility may be expected even at a low temperature around a room temperature.
Meanwhile, it is known that the heat resistance of the oxide semiconductor is not sufficient, and oxygen, zinc, or the like is detached in heating treatment during manufacture process of the TFT and the lattice defect is formed. The lattice defect results in formation of an electrically-shallow impurity level, and causes low resistance of the oxide semiconductor layer. Thus, the operation of the oxide semiconductor becomes normally-on type, that is, depression type in which a drain current flows without applying a gate voltage. The defect level is increased, the threshold voltage is reduced, and the leakage current is increased.
By such lattice defect, induction of a carrier in zinc oxide to become the active layer is inhibited, and the carrier concentration is reduced. The reduction of the carrier concentration causes a reduction of electric conductivity of the active layer, and has an influence on electron mobility and current transfer characteristics (for example, subthreshold characteristics and the threshold voltage) of the thin film transistor.
Thus, in the past, for example, it has been proposed that a gate insulating layer in contact with a channel layer of oxide semiconductor is composed of amorphous aluminum oxide (Al2O3), and the defect level of an interface is reduced (for example, Japanese Publication Patent No. 3913756).